Spacing between divs in text?

From a semantic point of view, using a floating div followed by an anchor isn't the best choice possible of tags in this case.
Personally, I would replace that div with a span, then you wouldn't need to float them, and so word spacing would work as usual..

Try this:

.replytolink {
    padding-left:5px;
    float: left;
}

Example: http://jsfiddle.net/RTFb7/

Add a padding-right to #replyto:

#replyto {
        float: left;
        padding-right:5px;
}

Add a space?

printf( '<div id="replyto">Reply to </div><a class="replytolink" href="%1$s">%2$s %3$s</a>', $parent_link, $parent->comment_author, $parent->comment_date );

Or add padding-right: 2em on #replyto.

Solution by Lucius:

printf( '<span>reply to <a href="%1$s">%2$s %3$s</a></span>', $parent_link, $parent->comment_author, $parent->comment_date );

Neat. No CSS is required. Give the <span> tag an ID if needed. The extra blank space after "reply to " is now output.